I bought a dwarf iPhone on eBay

So I bought this iPhone on eBay 6 months ago on tmobile. I called them and they activated it.
Suddenly 1 week ago, they deactivated my phone. I called and they told me that the first owner didn't pay.
So now what should I do?

Sue the first owner in court.

Take it up with the first owner and t-mobile and see how it can be fixed.